problem in copying

whenever i try to copy any movie from a DVD at first it works properly but after sometimes (after copying some files) it shows "CANNOT COPY: DATA ERROR (CYCLIC REDUNDANCY CHECK)". after 2-3 days if i copy that particular file at first it works then after sometimes the message pops up like before.... and this happens in case of almost all DVDs... what am i supposed to do???? plzz help!!!